The fabrication of traditional photovoltaic devices (PV cells), as well as displays and other optoelectronic devices, require numerous high temperature processes. Most of the common such devices are built on glass substrates. Recent advances though on flexible photovoltaic devices include the utilization of metallic substrates such as stainless steel, for the efficient fabrication of flexible solar cells over large areas. These photovoltaic devices are used in numerous terrestrial applications as alternatives to the conventional rigid cells built on glass substrates. Similarly, flexible displays have started to be fabricated on stainless steel substrates. Other technologies such as thin film batteries also can be developed on stainless steel substrates. Metallic substrates such as stainless steel offer numerous advantages. They are commercially available as rolls of various thickness down to 25 μm, and therefore can be processed in a continuous mode that is usually more cost effective than the batch mode. They can retain their dimensional stability with minimum change during the high processing temperatures needed for device fabrication. They can be made to be magnetic, and thus are amenable to continuous processing enabled by magnetic fields. The following publications may be referred to on the uses of stainless steel substrates in optoelectronics, energy and other fields: (i) Cannella, et. al., “Flexible Stainless-Steel Substrates” Information Display June 2005, page 24; (ii) Kessler, et. al., “Monolithically Integrated Cu(IN, Ga)Se2 Thin-Film Solar Modules on Flexible Polymer and Metal Foils”, 19th European Photovoltaic Solar Energy Conference, Jun. 7-11, 2004, Paris, France, page 1702: (iii) Baojie, Y., et. al. “High Efficiency Triple-Junction Solar Cells with Hydrogenated Nanocrystalline Silicon Bottom Cell”, Conference Record of the IEEE Photovoltaic Specialists Conference (2005), 31st, pages 1456-1459; and (iv) Beernink, et. al., “Ultralight amorphous silicon alloy photovoltaic modules for space applications”, Materials Research Society Symposium Proceedings (2002), 730 (Materials for Energy Storage Generation and Transport), Pages 193-198.
Some of the disadvantages of stainless steel are its high specific gravity over that of plastic substrates, its non-transparency, and its low surface roughness unless it is polished by expensive chemical-mechanical polishing (CMP) techniques. For space photovoltaic applications, it is desirable to have as a substrate a high temperature plastic film with low specific gravity, good transparency, and a smooth surface. In display applications, it is desirable to have optical transparency and smooth surfaces. The following publications may be referred to on the use of polyimide substrates for flexible PV devices: (i) Beernink, K. J., et. al., “High specific power amorphous silicon alloy photovoltaic modules”, Conference Record of the IEEE Photovoltaic Specialists Conference (2002), 29th, pages 998-1001; (ii) Guha, S., et. al., “Amorphous silicon alloy solar cells for space applications.”, European Commission, [Report] EUR (1998), (EUR 18656, 2nd World Conference on Photovoltaic Solar Energy Conversion, 1988, Volume III, pages 3609-3613; and (iii) U.S. Pat. No. 6,3001,158 (Oct. 9, 2001) on methods of fabricating thin film solar cells and thin film flexible circuit boards. In addition to their lightweight, common polymeric substrates are amenable to monolithic integration of the devices since they are insulating.
One variation of the current process that has been considered by those in the industry is a photovoltaic cell on a polyimide film that is cast onto a stainless steel foil. The polyimide film with the photovoltaic cell is then peeled off of the stainless steel foil at the end of the process. This strategy offers two advantages. First, the presence of stainless steel as an intimate support of the polymeric substrate has the potential to confine in part the deformations of the polymer during high temperature processing. Also the stainless steel substrate, if magnetic, can allow the use of a magnetic field for continuous roll-to-roll processing. In other words, it allows the use of the same equipment and process that are used for the fabrication of devices on stainless steel substrate alone. Second, the use of the dielectric polyimide film allows the subsequent monolithic integration of the devices in a cost effective manner. A third economic advantage is the potential recycling of the stainless steel.
However this modification has yet to become a viable commercial reality because (i) peeling of polyimide films from an unreleased stainless steel substrate requires a peeling force on the order of 170 g/cm, which will damage fragile photovoltaic devices on top of the film. In addition, the current release coating materials used in the industry such as polytetrafluoroethylene (PTFE), polyethylene adipate (PEA), and silicones of low crosslink density, cannot sustain a temperature of 425° C. that is the highest temperature polyimide films are subjected to during the process. This temperature is mandatory to obtain an acceptable efficiency measured from CIGS (Copper Indium Gallium Diselenide) cells on polyimide film. Similarly, CdTe-based PV cells require temperatures of the same magnitude for their fabrication. Devices that use polycrystalline silicon (e.g. thin film transistors), microcrystalline silicon, and amorphous silicon (PV-cells) require a range of high temperature processing from 280° C. and above. The present invention is a solution to these problems enabling the above mentioned process modifications to become a viable commercial reality for the electronic device manufacturing industry.